Doing this because I am bored. IMO, chances of veterans returning for SSB5 (initial roster, not including DLC). This assumes there are no more DLCs (which isn't the case, but it's just a hypothetical) and Sakurai returns to direct/has significant influence over the roster:
100%
75%
50%
37.5%
25%
12.5%
0%
If Sakurai does not have significant influence over the next game's roster, I would place Dark Pit's chances of returning at 25%. If Pichu becomes DLC this game, I would place him at a 12.5% of returning (or if he receives considerable decloning and is beloved, at 25%). If Namco Bandai returns to develop the next game, Pac-Man shoots up to 50%. Also worth noting that third-parties by default do not go above 50% since we have no idea if they will return for the next game or not. If they do, Sonic goes up to 75%, Mega Man, Pac-Man (without Namco's involvement), and Ryu goes to 50%, and Snake climbs up to 25%.